1. Field of the Invention
The present invention relates generally to the technical field of motive power systems for automotive vehicles, and, more particularly, to automotive-vehicle motive power systems that convert stored thermal energy into electricity for energizing electric-vehicle driving motors.
2. Description of the Prior Art
Present and anticipated air-pollution regulations indicate that emission-free vehicles will be operating in major urban areas in the foreseeable future. Presently, electricity appears to be the only possible energy source to power an emission-free vehicle. A significant difficulty with presently proposed electric vehicles is that they employ batteries which currently have an unsatisfactorily low energy density. The energy density of presently available batteries is so low that a battery-powered vehicle intended to travel 400 miles between battery recharging can carry nothing other than the batteries which power the vehicle's operation. Consequently, producing a useful battery-powered vehicle with a 400-mile cruising range requires a major breakthrough in battery technology.
For scores of years in various industrial settings steam-powered locomotives have operated which lack any ability for heating water to make steam. Rather, such locomotives are powered by stored thermal energy. The power source carried by these locomotives is a heavily insulated water tank which is periodically filled with superheated water. This superheated water provides the locomotive's power source. The energy density of such superheated water is so great that it has an energy density comparable to that of present electrical batteries.
Employing hot water as a thermal-energy-storage material, U.S. Pat. No. 5,385,214 which issued Jan. 31, 1995 on an application filed by John E. Spurgeon ("the Spurgeon patent"), discloses a motor vehicle having a "heat battery" which supplies high-pressure steam to a conventional heat engine that includes a steam turbine. The Spurgeon patent expressly discloses that the heat battery stores hot water at a temperature of 374 degrees centigrade (".degree.C."), i.e. 705 degrees Fahrenheit (".degree.F."), and at a pressure of 221 bar, i.e. 3205 pounds per square inch ("PSI"). The Spurgeon patent employs hot water at this particular temperature and pressure because under those conditions the material's heat capacity appears to be infinite.
A technical paper entitled "The Thermal Vehicle--A Pollution Free Concept" by Jack R. Kettler presented at the "Tenth Intersociety Energy Conversion Engineering Conference held Aug. 18-22, 1975 ("the Kettler paper"), compares thermal and electric vehicles, emphasizing system design performance. The Kettler paper also compares alternative types of heat engines and thermal-storage materials. The Kettler paper reports that most automotive gas-turbine-engine studies have dealt with internal-combustion gas turbines. It comments that these studies are not pertinent to the thermal vehicle since they used a 1900.degree. F. turbine inlet temperature, which would be too high for a gas turbine if its working fluid were to be heated by an external source. The Kettler paper further reports that a closed-cycle gas turbine operating with external combustion had been designed with a turbine inlet temperature of 1500.degree. F.
The Garrett Fluid Systems Division of Allied-Signal Aerospace Company has, over many years, developed various closed-Brayton-cycle power systems which have used a quantity of argon gas, continuously circulating in a closed loop, as a working fluid. Various heat sources including an electrical heat source, a diesel-fueled heat source, and a CO.sub.60 heat source have been used with these closed-Brayton-cycle power systems.
A technical paper entitled "Power From Thermal Energy Storage Systems" by Worth H. Percival and Michael Tsou presented at the Society of Automotive Engineers Combined National Fuels and Lubricants, Powerplant and Transportation Meetings, Oct. 29 through Nov. 1, 1962, (the Percival paper"), reports various proposals that have been made for using stored thermal energy. For example the Percival paper reports a proposal for electrically heating crushed stone during off-load hours, and then subsequently using the stored heat for space heating when electrical power system load is high. The Percival paper also reports that heat stored in refractory pebbles or spheres at temperatures as high as 3000.degree. F. has been used for brief intervals in heating air or other gases to a high temperature in high-speed wind tunnels. In considering various alternatives for converting stored heat into mechanical energy, the Percival paper states that, in comparison with the Stirling engine's efficiency of approximately 40%, it is questionable whether a Brayton-cycle gas turbine will ever reach an efficiency of 40%. The Percival paper states unequivocally that the Brayton-cycle gas turbine "can not hope to compete on the basis of total integrated work available from [a thermal energy] storage tank compared to the Rankine cycle steam cycle or Stirling engine." The Percival paper also states that in converting stored thermal energy into mechanical energy, a closed-Brayton-cycle turbine achieves higher efficiency than an open-Brayton-cycle turbine.